1. Field of the Invention
Broadly speaking, this invention relates to the generation of electrical energy. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for maximizing the power output of an electrical generator into an electric utility net which is driven by a source of variable power and speed.
2. Discussion of the Prior Art
Recent events have focused attention on alternate techniques for generating electricity. Among the systems that have been considered are wind turbines, hydroelectric generators and wave and tidal energy conversion devices. All of the above are characterized by the fact that the source which drives the generator is subject to long and short term variations in speed and power which, in turn, affects the frequency and power output of the generator.
By way of example, the following discussion will address the specific problem of generating electricity by the use of wind turbines. One skilled in the art will appreciate, however, that the invention is not so limited and has equal application to any field where the driving source is subject to fluctuation including, but not limited to, the aforementioned hydroelectric, wave, and tidal energy generation systems.
In its broadest aspect, the problem to be solved is the generation of stable electric power from an unstable energy source. According to the invention, the solution to this problem comprises an externally-controlled, electronic, differential-frequency corrector that may advantageously be mounted within the generator housing itself.
As is well known, the development of practical, wind-powered generating systems create problems which are simply not encountered in the development of conventional power systems. Some of these problems arise from:
1. the natural instability of the wind; and
2. the relationship between the velocity of the tip of a turbine propeller and the wind velocity and how this relationship affects the maximum energy that may be captured from the wind.
The first problem affects the nature and quality of the electricity produced while the second problem affects the cost of the electricity produced. Because of these factors, it quickly becomes apparent that, in order to achieve the desired system optimization, it would be necessary to design a new kind of generator for use with wind turbines.
Now, in the design of such a generator system one must not forget that, for reasons for performance and cost effectiveness, the synchronous generator has already been determined to be the best choice for wind-driven power systems. For example, a present NASA-sponsored, wind turbine program calls for an 1800 r.p.m., wound-rotor, salient-pole generator. When operating in parallel with an existing power system this generator must, of course, stay in synchronism with the existing system. Thus, the shaft speed of the generator and, hence, the speed of the turbine, must remain constant over all ranges of operating wind speeds.
Now, the power which is delivered by a wind turbine to a commercial power system, which runs at constant speed, is determined by the torque produced by the turbine. The turbine, in turn, is typically controlled by a power command signal which is fed to the turbine propeller pitch servos. Because of stability considerations, this control loop must be operated with a limited bandwidth and, thus, is not capable of responding adequately to transient power surges.
The above factors result in the following conditions:
1. system operation at a constant speed prevents the maximum energy capture from the wind because the ratio of the velocity of the tip of the turbine propeller to the wind velocity is maximized for only one wind velocity; and
2. the slow response of the turbine blade pitch control loop prevents suppression of torque fluctuations arising from wind gusts, tower shadow, or unwanted motion of the propellers. The resultant power surges in and out of the public power system may cause detrimental disturbances in the network grid.